I use a Canon 50mm macro for some shots but to be honest unless you going for just the head/leg/eye etc. its overkill and you miss out a lot due to the heavy depth of field ie. blurring everything that is not in your very small focal range.
Most of the time you just want a decent midrange lens and good old fashion sunlight (diffused not direct). Flashes and other electric light are going to mess with your colour (and that's the whole point with cham photography
Rather use your standard lens on a tripod and zoom in. Use slower shutter speed if your lighting is low.
Fancy lenses are great but unnecessary.
My best tip to an aspiring amateur is: switch your camera to "M" (manual) as often as possible and look at each pic after you take it and adjust.
All the options may seem daunting but all you really need to know on your xsi Rebel is use the wheel on the right hand side to change your shutter speed until its not to dark or bright.
After awhile you will start to know what the numbers mean and what different lighting will need without having to guess so much.
You will find the kit lense (18-55mm f3.5/5.6) to work well for most shots.
I shoot with the kit lense alot for regular portraits, If I want to get close up or macro I use my Canon 100mm f2.8 Macro lens. If you are not currently using the manual mode on your camera your really missing out on the full potential of your camera. Shooting in natural sun light will provide the best results, however many of us use flashes when needed. A good flash set up for the XSi is a Canon 580exII or a 430exII with either an omni bounce or a softbox.